1. Field of the Invention
This invention relates to a photosensitive printing plate forming material and more particularly a material which is exposed by the vacuum contact method.
2. Description of the Prior Art
In the exposure of photosensitive printing plates through an image-bearing original, complete contact of the image-bearing original with the surface of a photosensitive layer of the photosensitive printing plate or with the surface of a resin layer which may be formed on top of the photosensitive layer (to be inclusively referred to simply as the surface of a photosensitive printing plate) has previously been secured by disposing the photosensitive printing plate and the image-bearing original in superposed condition between a rubber sheet and a press-bonded glass and evacuating the space between the rubber sheet and the press-bonded glass (hereafter this method is referred to as the "vacuum-contacting method"). Here and throughout the balance of this disclosure the term "photosensitive printing plate" will be used with reference to the presensitized material from which the plate is formed as opposed to the plate itself. A conventional photosensitive printing plate has a smooth surface. When an image-bearing original is contacted intimately with the surface of the printing plate by the vacuum-contacting method, contact is first accomplished at the peripheral portion of the printing plate. Hence, evacuation of the central portion is impeded, and extremely long periods of time are required to bring the image-bearing original into complete contact with the entire surface of the photosensitive printing plate. When the printing plate is exposed imagewise while contacting of the image-bearing original is incomplete, a clear image is not obtained in the portions where contact is incomplete and clear printed copies cannot be obtained. Since the extended period of time required for contacting reduces the efficiency of the plate-making operation, it has long been desired to shorten this time.
British Pat. No. 1,495,361 discloses a photosensitive printing plate obtained by shortening such contact time. By providing a matting layer removable at the time of development as the uppermost layer of the photosensitive printing plate, the time required for vacuum-contact can be markedly shortened. It gives as one example of the matting layer a layer composed of a fine particulate material as a matting agent dispersed in a resin as a binder. Specific examples of the resin include water-soluble resins such as gum arabic, cellulose and polyvinyl alcohol, and hydrophobic resins such as epoxy resins and phenolic resins.
It has been found, however, that in the photosensitive printing plate containing the water-soluble resin as a binder of the matting layer, the matting layer absorbs moisture in the air which in turn causes deterioration of the photosensitive layer. It has also been found that the photosensitive printing plate containing the aforesaid hydrophobic resin as a binder for the matting layer requires a longer developing time than does a photosensitive printing plate having no matting layer.